O-Level Chemical Calculations: worked solution

Question

Using the ions listed (CO3^2-, NH4+, OH-, SO4^2-, NO3-) together with the Periodic Table, write the formula of each compound: (i) calcium nitrate (ii) sodium carbonate (iii) aluminium sulfate

A compound is electrically neutral, so the total positive charge must cancel the total negative charge exactly. Step 1 - get the charge on each metal ion from its Group in the Periodic Table. Group I -> 1+, Group II -> 2+, Group III -> 3+. So calcium (Group II) is Ca2+, sodium (Group I) is Na+, aluminium (Group III) is Al3+. The negative ions are the ones listed: NO3-, CO3²-, SO4²-. (i) calcium nitrate: Ca2+ carries 2+, NO3- carries 1-. One Ca2+ needs two NO3- so the charges cancel: 2+ and 2 x 1- = 2-. NO3 is a group of atoms, so it must be bracketed before the 2. Formula: Ca(NO3)2 (ii) sodium carbonate: Na+ carries 1+, CO3²- carries 2-. One CO3²- needs two Na+: 2 x 1+ = 2+ cancels the 2-. Na is a single atom, so no brackets are needed. Formula: Na2CO3 (iii) aluminium sulfate: Al3+ carries 3+, SO4²- carries 2-. The smallest total charge that both 3 and 2 divide into is 6. So take 2 Al3+ (2 x 3+ = 6+) and 3 SO4²- (3 x 2- = 6-). SO4 is a group of atoms, so bracket it before the 3. Formula: Al2(SO4)3